Recently, portable telephones and portable information terminals have rapidly spread due to the development of mobile communication systems. Thus, there is an increasing demand for RF filters, particularly, those having a compact size and a sharp characteristic. The filters are required to generate an attenuation pole and have sufficient suppression over a broad band in order to handle a new communication system. Such filters may be a ladder filter in which multiple resonators are connected. A duplexer or module equipped with filters is required to have transmission and reception filters configured so that the transmission filter has high suppression in the pass band of the reception filter and the reception filter has high suppression in the pass band of the transmission filter. There is a proposal of a filter equipped with an inductor in order to adjust the filter characteristics.
Japanese Patent Application Publication No. 6-350390 (Document 1) discloses a resonator with which an LC resonance circuit is connected in parallel. Japanese Patent Application Publication No. 11-55067 (Document 2) discloses a ladder filter to which an inductor is connected. The arts disclosed in Documents 1 and 2 are capable of generating an attenuation pole outside of the pass band.